In mining operations, it is common to provide a walking-prop system which comprises a skid which can be advanced along the floor of the tunnel substantially in the cadence of advance and the mining or excavating machine to follow the recession of the mining face, the walking prop having hydraulic rams which can press a roof cap against the roof of the tunnel. The roof cap generally extends forwardly of the skid and the ram to have a cantilevered portion which can overhang the mining machine and protect it from falling rock from the tunnel roof The rear side or goaf side of the prop can be provided with a shield, hereinafter referred to as a goaf-side shield, which can be articulated to the roof cap and to the skid.
The mechanism for advancing the skid, which can be provided in two skid parts laterally spaced from one another, can include a guide-rod assembly extending in the direction of displacement of the skid and anchored, for example, to a conveyor for the mining debris and the excavating machine, and a stepping hydraulic piston-and-cylinder assembly which can advance the skid against the guide-rod assembly.
In conventional operations, the mining machine and conveyor is advanced to follow the cutting away of the mining face or wall, drawing the guide-rod assembly in the forward direction.
When the guide-rod assembly is fully extended forwardly, the rams are retracted somewhat so that the prop is no longer braced against the roof, and the stepping mechanism is actuated to shift the skid along the guide-rod assembly, thereby causing the prop to move in the direction of the conveyor and mining machine. The hydraulic ram then presses the cap against the roof so that the prop is braced between the floor and the roof. A series of props can be provided along the wall which is mined in long-wall mining, as may be desired.
The device may be provided with a skid-lifting cylinder which causes the front end or leading end of the skid to tilt upwardly for advance of the skid, thereby preventing interference with such advance by irregularity in the ground.
When reference is made herein to a horizontal skid, we preferably intend to describe a divided skid, namely, a skid which is composed of two laterally-spaced but parallel parts which may be interconnected by a traverse. The stepping cylinder can be located between the two parts of the skid and, just as the reference to a horizontal skid is intended to include one-part and two-part skids, references to the cap, and the goaf-side shield may refer to one-piece and two-piece caps and shields. The journals and articulations of these elements can be traverses as well.
In the past, the lifting mechanism have been designed to lift the leading end of the one-piece skid or to lift the leading ends or the separate skid parts when the latter together form the skid.
In earlier systems as well, the guide-rod assembly had a goaf-side end pivotally connected by a traverse to the stepping cylinder assembly. Because of spatial requirements, the skid-lifting cylinder arrangement was horizontally oriented in the region of the ram and connected via a lever transmission to the guide-rod assembly. As a result of this orientation, the lifting movement was effected with a relatively short lever arm and unsatisfactory force-transmission characteristics. As a result, the mechanism was strained and could suffer breakdown, the lifting stroke was limited and by and large, the system was found to be unsatisfactory.